date beyond which carbon 14 decay is/is not useful.
However, given that the half life of carbon 14 is 5730 years, then there really isn't much carbon 14 left in a sample that is 40,000 years old.
These collisions create secondary cosmic rays in the form of energentic neutrons.
When these neutrons collide with nitrogen-14 in the atmosphere carbon-14 can be created.
So by measuring the C-14 level we work out how many half lives old the sample is and therefore how old it is.
The use of various radioisotopes allows the dating of biological and geological samples with a high degree of accuracy.
The half life of carbon-14 is about 5,700 years, so if we measure the proportion of C-14 in a sample and discover it's half a part per trillion, i.e.
half the original level, we know the sample is around one half life or 5,700 years old.
Carbon-14 is created from nitrogen-14 in the upper atmosphere of the earth.
Radiation from the sun collides with atoms in the atmosphere.